Soil stabilisation and solidification (S/S) is a civil engineering based remediation technique where contaminated soil is mixed on site with cementitious materials and transformed in a stable and durable product. It simply locks in the contamination. S/S was first used in the 1950s with the successful treatment of nuclear wastes. Its use for the treatment of non-nuclear wastes was established in the US in the 1980s in order to limit the disposal of untreated hazardous wastes to landfill.
The success of the technique in containing potential contaminants combines with its ability to improve soil engineering properties and to overcome the problems excavating and then finding a disposal site. An unsophisticated 'dig and dump' approach is unsustainable. It has the potential to spread the contaminants during excavation and rather than deal with the problem merely shifts it elsewhere to fast disappearing landfill sites.
The benefits of S/S have resulted in it being increasingly used to reclaim contaminated land in the US, Europe and Japan. Indeed, in the US, S/S is used for around 25% of remediation carried out under the Comprehensive Environmental Response Compensation and Liability Act (CERCLA) superfund programme, set up to address situations where hazardous substances pose potential danger to human health and environment. It is also recognised as being an approved technique with Best Demonstrable Available Technology (BDAT) status for the treatment of materials contaminated with metals.
As a process, S/S is a relatively simple civil engineering based technique that involves the controlled addition and mixing of inorganic hydraulic powders or binders with contaminated soils to form a new solid in which the contaminants are rendered practically immobile and, therefore, non-leachable. Although not removed or destroyed, the contaminants' potential to pollute is removed.
Adding a binder triggers two separate yet simultaneously acting mechanisms: stabilisation and solidification.
Stabilisation is the chemical modification of contaminants so that they are more stable and significantly less mobile. This involves a number of processes such as absorption, reprecipitation of more soluble forms and the formation of low solubility salts.
Solidification occurs via the physical alteration of the soil material to make it more stable and less prone to being affected by external agents that may mobilise the contaminants such as percolating groundwater passing through the soil. For example, solidification may involve the formation of an indurate (cemented) mass for low permeability.
In the UK, the most common used principal binders include:
cement; lime; ground granulated blast furnace slag; fly ash and pulverised fuel ash. The principal binders are used either separately or in combination and do most of the work in managing the contamination. Additives are used to modify and fine tune the efficacy of the process. These additives may be proprietary or more generic reagents.
The list of possible additives is extensive and includes: inter alia; pH/Redox modifiers; wetting agents; flocculants and sorbents. The binders and additives are mixed into the soil using a variety of plant and methods.
Among these are rotovatortype plant; modified excavation plant; augers and sub-surface injection; single pugmill or a 'treatment train' that involves a range of treatment processes.
The mixing approach is determined by the soil and contamination type and depth. Each site is different in terms of contamination, nature of the soils and the remedial objectives. Such is the versatility of S/S that the technique is fully able to deliver a bespoke solution for each remediation project.
S/S is compatible with UK and EU legislation. It satisfies the objectives of sustainability, allows the on site recycling of materials and is well suited to the redevelopment of brownfield sites, which often have complex contamination profiles and physically difficult conditions.
Moreover, it offers a pragmatic and cost effective alternative to landfilling. It is this benefit that will encourage greater use of S/S in the UK. The EU Landfill Directive 1999, implemented in the UK as the Landfill (England and Wales) Regulations 2002 and Landfill (Scotland) Directive 2002, calls for a profound change in current landfill practice.
The Environment Agency fully supports the use of processed based technologies such as S/S as a best practice technique and views it as an important technique for the pretreatment of wastes prior to disposal. Pre-treatment is a core requirement for landfilling wastes under the EU directive and subsequent UK regulations.
The implementation of the European Landfill Directive will result in massive change to landfill practice and the use of 'dig and dump' as the preferred land remediation technique is likely to change in favour of technologies that manage the contamination on site.
S/S as a straightforward civil engineering approach looks set to become a mainstream remediation technique due to its ability to deal with a wide range of contaminants, its simplicity and its relative cheapness.
l The Concrete Centre, together with the British Cement Association, is to publish later this year The essential guide to stabilisation/solidification for the remediation of brownfield land using cement and lime. For further details contact the concrete helpline on 0 700 5 400 400, 0 700 5 CONCRETE.
Alan Bromage is head of civil engineering for The Concrete Centre.